Self-assembly has emerged as a powerful fabrication technology for fabricating macroelectronic devices. Macrofabrication technologies typically integrate a large number of various functional components over areas exceeding the size of a typical semiconductor wafer and do so in a cost-effective and time-efficient fashion. A typical self-assembly macrofabrication technique includes functional devices that are batch microfabricated (for example, on a semiconductor substrate) and released to yield a collection of free-standing components. These components are then allowed to self-assemble onto a template, for example, on a plastic substrate, to yield a functional macroelectronic system. Because self-assembly is an inherently parallel construction method, it allows for cost-effective and time-efficient integration of a large number of functional components onto both conventional (e.g., semiconductor) and unconventional (e.g., plastic) substrates.
An additional benefit of self-assembled macrofabrication is that it allows for the integration of components made from incompatible microfabrication processes (e.g., light-emitting diodes made in compound semiconductor substrates and silicon transistors) onto nonplanar and/or flexible substrates.
The components of a self-assembly based macroelectronic fabrication system typically include (1) the development of fabrication processes that generate free-standing functional components; (2) the implementation of recognition/binding capabilities to guide the components to bind in the correct location; and (3) the determination of self-assembly procedures/conditions that result in a final assembled system with a high yield of components in correct locations. An exemplary fluidic self-assembly method is disclosed in International Patent Application No. PCT/US2007/072038, filed Jun. 25, 2007, which is hereby incorporated by reference in its entirety. Additionally, fluidic self-assembly has been used to fabricate macro-scale electronics comprising an integrated optical analysis system in International Application No. PCT/US2008/050104, which is also hereby incorporated by reference in its entirety.
Briefly, the above-referenced international patent applications describe a method for self-assembly that accomplishes the assembly process in one step, obviating or mitigating the need for post-processing of an assembled macro-electronic device. Microcomponents are fabricated having a particular shape, and a template with embedded interconnects is fabricated having recessed binding sites that are sized to receive particular microcomponent types. The binding sites include a low melting point alloy for electrically connecting received microcomponents to the interconnect network. The template is placed in a liquid, and the microcomponents are introduced to the liquid such that the microcomponents flow or slide along the template propelled by gravity and/or fluid-dynamic forces and some of them are received into the binding sites, and retained by capillary forces. The liquid is heated before or after introduction of the microcomponents to melt the alloy. The fluid and/or template are then cooled to harden the alloy, binding the microcomponents.
Prior macro-scale self-assembly techniques have produced macroelectronic devices having structures such as light-emitting diodes, photosensors, and transistors, to name a few. One aspect of previous macroelectronic self-assembled devices is the limitation of fabrication to patterning features on one side of a device. In a typical fabrication procedure for components for macro-scale self-assembly, micron-scale devices are patterned on a substrate and then released for self-assembly.
Given traditional microelectronic fabrication techniques, it is not surprising that only one side of the micron scale devices can be patterned, because only the top side of a substrate is typically processed in microfabrication. This remains true when fabricating micron-scale devices for macroelectronic self-assembly in that only the top side of a device is typically patterned and processed to create the device structure. The eventual bottom side of the device is buried within (or adjacent to) the carrier substrate during processing, and the bottom of the device is only revealed once the devices are released from the carrier substrate, at which point the devices are individually articulated and batch processing of a plurality of such devices would be nearly impossible.
A processing technique enabling the patterning of both the top and bottom sides of micron-scale devices for fluidic self-assembly of macroelectronics would enable more complex devices (both on the micro and macro-scale) and increase the number of currently available types of device structures. By allowing more complex features to be integrated onto macroelectronic devices, the functions of such devices will potentially be improved, expanded, and enhanced.
Contact lenses may be one product that would benefit from the integration of micron-scale devices into macroelectronic systems. Contact lenses could become complex systems with circuitry, sensors, memory, and telecommunications used to track key biomarkers in tears or to show information to the wearer. Concentrations of molecules such as glucose, lactate, or cholesterol on the surface of the eye could be measured, stored, and then communicated to a handheld reader or a mobile phone. Contact lens sensors have been used to measure eye movement, corneal temperature, blood oxygen, tear glucose concentration, and intraocular pressure. However, few have employed electronic sensing and wireless data readout, and previous work demonstrated limited operating distances. A functional contact lens system requires a source of power. All prior work in this area has used either a wired approach to transfer power to the contact lens or has taken advantage of Radio Frequency (RF) power harvesting from a nearby source. The wired approach is helpful for validation of system components but cannot yield a stand-alone contact lens. RF power transmission requires an RF source in the vicinity of the contact lens and suffers from the low efficiency of small antennas that can fit within the form factor of a contact lens.
A supplemental power source (on lens) can increase the RF read range, as well as allow sensor sampling when RF power is not present.
Relatedly, miniaturized solar cells have been developed. For example, solar cells on 50 μm device-layer thickness silicon-on-insulator (SOI) wafers with isolation trenches etched to the buried oxide have been fabricated. The cells were connected in arrays with an estimated 14.3% efficiency at AM2.0. Similarly, another report used SOI wafers, isolation trenches, and arrays, but with device thicknesses of 5 and 10 μm. In both cases the cells remained on the handle wafer for mechanical stability and, hence, were not freestanding.
Diabetes is widely recognized as a leading cause of death and disability throughout the world, and the number of people diagnosed with diabetes mellitus is expected to increase dramatically in the next few decades. Diabetes management mainly concentrates on maintaining normal blood sugar levels through frequent glucose monitoring and the correct dosage and timing of insulin injections. Continuous glucose monitoring can help early diagnosis and effective control of diabetes complications.
An enzyme-based finger-pricking method is the most commonly used diabetic assessment. However, the procedure is invasive and inconvenient, requires patient compliance, and may cause infection during the blood sampling processes. An alternative method uses near-infrared spectroscopy and provides a noninvasive way to monitor the glucose level in the body. This method analyzes the light reflection or transmission spectrum in the fingertip to infer metabolic concentration. Due to challenges of interference with other biochemicals, poor signal strength, and calibration issues, this method is not sufficiently accurate for clinical use. Therefore, ongoing research focuses on the development of noninvasive and continuous glucose sensing.
Tear fluid is directly accessible on the eye and can be used as a chemical interface between a sensor and the human body. Tear fluid contains many biomarkers that are found in blood, such as glucose, cholesterol, sodium, and potassium. The glucose level in tear film is reported to be in the range of 0.1-0.6 millimoles per liter (mM), which is about ten times lower than the levels in blood.
Conventional contact lenses are transparent polymers placed on the eye to correct faulty vision and can simultaneously serve as a platform to directly access tear fluid. Integrating biosensors on a contact lens would provide a noninvasive way for continuously sensing metabolites in tear fluid. Contact-lens-mounted biosensors have been developed to measure eyelid pressure, tear glucose, and intraocular pressure. These sensors use inconvenient wired readout interfaces. Contact-lens functionality could be greatly expanded by creating heterogeneous systems with embedded electronics and wireless telemetry. Through integrating biological sensors and telemetry, an active contact lens could provide health professionals with a new tool for research studies and for diagnosing diseases without the need for lab chemistry or needles.